Energy use metrics

Some of the most commonly-used energy use metrics are explained in the table below.

Energy use intensity (EUI)

Energy use intensity (EUI) is a normalized quantitative benchmarking metric. The EUI for a building is calculated by taking the total energy consumed in one year (usually measured in kWh) and dividing it by the total floor space of the building. For example, if a 10,000 m2 building used 1,000,000 kWh over the year, its EUI would be 100 kWh/m2. It is important to consider different building uses and maybe compare buildings within the same category, i.e. all offices or lab facilities

While EUI is a commonly-used and sensible metric, it does have some limitations. Specifically, other factors such as occupancy, presence of air conditioning and weather or geographical aspects will all tend to influence EUI. Likewise, determining floor space is not necessarily a straightforward task where, for example, space is leased within a larger building.

To help improve this, sometimes a modified EUI is used, expressing energy use intensity by floor space and employees (kWh/m2/employee). Normalization of EUI to take into account weather conditions is also often required, since it is not reasonable to compare two similar buildings with significantly different external environments.

Site Energy1

Site energy measures the amount of energy consumed at a facility, which is derived from the electric, gas (debit) and renewable on-site generation (credit) such as PV. The primary advantage of this metric is that it is easy to measure and comprehend and puts the focus of efficiency at the location where building owners and operators have the most control: the building.

Source Energy2

Source energy includes the energy used offsite to generate and transport the energy, which is consumed at the building and site energy. Advantages are that it provides a full accounting of the impacts of the energy used (environmental, resources, etc.) and uncovers the "hidden" costs of energy consumed onsite. A disadvantage is that knowledge of secondary and primary energy coefficients (‘Well to Tank’ and ‘Transmission and Distribution’ losses) are required, including carbon factors for purchased power.

Energy per unit of output

This is a commonly used metric for manufacturing firms, expressing total energy use by amount of good produced (e.g. kWh consumption per kg of rubber, per laptop, etc.). This metric takes account of fluctuating production volumes, avoiding scenarios where a facility with falling production could be mistakenly identified as a facility performing well in terms of energy efficiency. Some climate change policies focus specifically on minimizing this metric3.

A similar approach can be adopted by service sector companies, whereby energy output is expressed by number of customers or amount of service provided.

Energy per unit of economic profit

Energy per unit of economic profit is similar in principal to the above but can be used where the unit of output may be variable or hard to define, for example in a factory that produces many different products that change frequently and require different amounts of energy to produce, or where an output is non-physical.

Heating Degree Days Heating Degree Days adjust energy consumption in-line with heating and cooling requirements based on site location and weather conditions.

References

1 Tristan Roberts, undated. ‘Energy Metrics for Buildings: a Primer’, in BuildingGreen. Available online

2 Tristan Roberts, undated. ‘Energy Metrics for Buildings: a Primer’, in BuildingGreen. Available online

3 Environment Agency, 2015. Climate Change Agreements Operations Manual, version 4. Available online